The invention relates to a temperature switching device for an electrotechnical part to be protected against an excess temperature due to malfunctioning of, for example, a motor such as, a motor for roller blind drives or the like, with a first bimetallic switch and an ohmic resistor connected in parallel thereto, so that, after opening the bimetallic switch, as a result of an excess temperature, the flow of current takes place across the ohmic resistor, which produces heat and, as a result, the bimetallic switch is kept open even in the case of the cooling of the excess temperature initially triggering the switching processs due to a malfunction of the electrotechnical part to be protected, as a result of the evolution of heat of the ohmic resistor.
Self-holding temperature switching devices of the aforementioned type have been proposed and are self-holding because they are kept open by the resistor connected in parallel to the switch mechanism as a result of the heat evolution thereof following an opening of the switching device. In the case of use, particularly in connection with motors, a problem can arise with ohmic resistors in that, if the switching devices are encapsulated, a high temperature occurs due to the ambient temperature leading to the disconnection of the switch mechanism in addition to the resulting temperature produced by the parallel connected resistor in the switching device. This problem causes irreversible damage to the switching bimetallic element and/or insulating materials, such as, insulating varnish or coatings of wire, as well as the device to be protected. Moreover, sealing compounds, etc. are softened or damaged and even soldered joints can undergo a strength reduction. This problem can in particular occur with roller blind drives, because, even with very low external temperatures with drive and subsequent disconnection of the thermal switch, the thermal switch must be kept open by the parallel connected resistor, that is, the parallel connected resistor must produce a considerable amount of heat. However, the disconnection temperature must not be chosen too low, because otherwise the thermal switch would open in the case of heating, due to, for example, insulation, and therefore would render the drive inoperative.
In addition, self-holding temperature switching devices are known, in which the parallel connected resistor is a PTC and not ohmic resistor; however, these self-holding temperature switching devices are much more expensive than ohmic resistors, such as e.g. thick film resistors, etc.
The aim underlying the invention essentially resides in providing a switching device of the afore-mentioned type with an ohmic resistor connected in parallel to a thermal switch or, more generally, a temperature sensitive electrical switch or thermal switch, in which damage or deterioration to the thermal switch is reliably avoided following the disconnection thereof due to an ambient excess temperature.
According to the invention, a temperature switching device for an electrotechnical part to be protected against an excess temperature due to a fault, such as, for example, for roller blind drives or the like, provided which includes a first bimetallic switch and an ohmic resistor connected in parallel thereto, so that after opening the bimetallic switch as a result of an excess temperature the flow of current takes place across the ohmic resistor producing heat and, as a result the bimetallic switch is kept open even upon a cooling of the excess temperature initially triggering the switching process with at least the ohmic resistor being connected in series with a further, automatically resettable thermal switch having an opening temperature above that of the first thermal switch, but well below temperatures causing irreversible damage to bimetallic elements, and with the two thermal switchs being in close thermal contact.
According to the inventive construction, after the disconnection of the first thermal switch connected in parallel with the resistor and the following current flow through the parallel connected resistor, which brings about a temperature increase, which initially increase the excess temperature leading to the disconnection of the first thermal switch, the second thermal switch is opened and interrupts the current flow to the resistor so that the resistor creates no further heating power and produces no furher heat, whereby inadmissible excess temperatures which can damage bimetallic disks, sealing compounds or soldered joints are avoided.
According to preferred developments, the additional thermal switch is connected in series both with the first bimetallic thermal switch and with the ohmic resistor or the additional thermal switch is in parallel to the first thermal switch. According to another preferred development the two thermal switches are juxtaposed on a common heat conducting support.
According to another development of the invention a contact part of each switch its casing and the support are constructed in a current-conducting manner, while the support has facing clips bent in U-shaped manner over the thermal switches, as well as further clips securing the same.
According to further developments of the invention, the two thermal switches are axially superimposed or one of the switches has a spacer and the switches are interconnected by a rivet as a mechanical holding and electrical connecting member.
Miniature thermal switches of different constructions have been proposed in, for example, GB-1 377 990, GB-1 394 612, DE-OS Nos. 24 32 901, 24 42 090, 24 42 397, 25 05 966, 25 11 214, 26 44 411, U.S. Pat. Nos. 4,306,211, 4,470,033 . For details reference should be made to the aforementioned publications, whose content is made into part of the disclosure of the present application. These proposed switches are very compact with a casing and a cup-shaped lower part and an upper part closing the latter. In the casing is arranged a switch mechanism having one or more contacts, which are centrally carried and switched by a bimetallic disk. Such a thermal switch has a dimension of a few millimeters typically a diameter of 5 to 8 mm and a height of less than 5 mm. If there is only one contact pair formed by a movable contact part and a stationary opposite contact part, the current flow can, in the simplest form, take place via the bimetallic disk, but preferably the current flow takes place by an additional spring snap disk to the cup-shaped casing. In the case of two movable contacts with a corresponding opposite contact, the opposite contact is carried by a common support member moved by the bimetallic disk and the current flow takes place directly. From the opposite contacts which are generally arranged on the fixed part, the current can be tapped via a current transmission element on one or more outer connecting contacts. Thermal switches, namely, temperature limiters are known, which only interrupt the electric contact and can either be manually switched back for restoring the electrical connection, or can be replaced. Thermal switches are known which, following a significant temperature change, are automatically switched back again and restore the electrical connection. It can then occur that following the cooling of the bimetallic disk the latter switches back again and restores the electrical connection, without removing the fault which, directly due to the increased current flow or indirectly, led to the increased temperature evolution and therefore to the separation of the electrical connection by the bimetallic disk. Thus, such switches provide for so-called control cycles of operation.
According to a preferred development, the inventive switches device is formed by a casing with a cup-shaped lower part and a top part, with the lower part containing a switch mechanism with at least one bimetallic disk and a movable contact part and which under the action of the bimetallic disk the movable contact part can be linked with a stationary opposite contact, so that an electrical connection is formed between the lower part, movable contact part and opposite contact part a permanent, higher ohmic, electrical connection through the parallel connected resistor part is provided between the opposite contact part and lower contact part. Thus, miniature thermal switches can be constructed as self-holding thermal switches. The small dimensions of such switches and the arrangement in a small, closed casing are retained and there is a more effective heat production for reliably keeping the switch open.
For the optimum solution of the problem, the solution not adopted is that of bridging by a high value resistor the outer connection contacts connected to the connecting wires, as has generally been done in known open switches in order to construct them in a self-holding manner. In the case of the inventive miniature switches this would lead to a considerable increase in the size of the overall unit, so that the latter could no longer be used for the intended purpose. It would also be disadvantageous in such a construction that the heat transfer from the high value resistor to the bimetallic disk would be extremely poor. Thus, the invention proposes an integrated solution with an integrated construction of the high value resistor in the existing casing.
According to a particularly preferred construction, the electrical resistor of the resistor part is adjustable. Thus, with a single resistor part for different switches which, in the case of the same construction, have bimetallic disk with different switching temperatures, a single high value resistor part can be used which, as a function of the bimetallic disk used and its switching temperature can be adjusted and adapted thereto. Preferably the resistor part is a metal or carbon resistor part guided in an arcuate or curved manner and, in particular the resistor part is stamped from a metal disk or the carbon resistor is in particular pressed as a coating onto a support and covered by a solidified glass melt. According to a particularly preferred construction, for adjustment purposes removable bridges are provided between individual portions of the resistor part. Thus, according to a preferred construction, the invention proposes that the insulating cover part forming a mechanical connection between the stationary opposite contact or the curent transmission member from the stationary opposite contact to an outer connecting contact part and the casing lower part, is constructed as a high value resistor element or is provided with such, so that there is a high value connection between the stationary opposite contact and the metal casing lower part.
The inventive switch can in particular be used at difficultly accessible points, where there is little space and/or contamination is to be feared. It can in particular be used within the windings of electric motors, or in the case of pressure upstream of lamps and in heaters. A particular advantage results through the combination of the encapsulated miniature construction and self-holding, in that the switch can replace manually resettable switches, due to the resetting device, which had relatively large space requirements and which can be reduced with the same level of security by using the inventive switch.